1. Field of the Invention
The present invention relates to a communication apparatus, a communication method, an image forming apparatus utilizing the same, a unit connected to the image forming apparatus and an image forming system.
2. Related Background Art
At first there will be briefly explained a noise elimination apparatus utilizing a digital noise filter of digital input. As disclosed for example in Japanese Patent Application Laid-Open No. 58-205327, there is already known a noise elimination apparatus utilizing a digital noise filter, in which the state of the input signal is monitored plural times at a predetermined time interval, and the state of the signal is captured only when, after a change in the state of the signal, the changed state continues for a predetermined time. Also there is disclosed, for example in Japanese Patent Application Laid-Open No. 53-142157, a noise elimination apparatus with a variable digital noise filter capable of arbitrarily setting the observation time for the continuation of the state of the signal after the change thereof. The variable digital noise filter is advantageous in that the time not sensing the noise can be arbitrarily set according to the magnitude and time of the noise to be eliminated. It is effective in case the noise situation is not clear in the location where the noise elimination apparatus is to be installed or in case the noise situation changes from time to time.
FIG. 10 is a block diagram showing an example of the noise elimination apparatus utilizing the variable digital noise filter. A noise elimination unit 114 is provided with a sampling unit 108 for sampling an input signal at the timing of an input reference clock, a change point detection unit 109 for detecting a point of change to a low logic level if the logic level prior to the point of observation is high, or to a high logic level if the logic level prior to the point of observation is low, and a same level continuation observation unit 110 for observing whether the changed logic level continues for a predetermined time after the change point is detected by the change point detection unit 110. The same level continuation observation unit 110 observes, in case the logic level changes from high to low, whether the low logic level continues for a predetermined time after the state change thereto, or, in case the logic level changes from low to high, whether the high logic level continues for a predetermined time after the state change thereto. It is thus rendered possible to identify whether the aforementioned change of the logic level has resulted from an actual signal or from a noise, because the noise shows rapid repetitions of high and low logic levels and cannot maintain, for the predetermined time, the changed logic level after the change thereof.
The noise elimination unit is further provided with a same level continuation observation time setting unit 111 for setting or changing the predetermined time for observing the same level continuation observation unit 110. Such unit allows to set an appropriate predetermined time in another system with a different reference clock or with a different noise environment, thereby achieving noise elimination more effectively. The noise elimination unit 114 is further provided with a data capture timing generation unit 112 for determining the timing of capturing the input signal in response to the result of the same level continuation observation unit 110, and a latch/hold unit 113 for latching or holding the actually sampled input signal in response to the result of the data capture timing generation unit 112. In case the input signal, after the change thereof, maintains a same logic level for the predetermined time, the latch/hold unit 113 latches the sampling data as effective data, after the lapse of the predetermined time. Otherwise it holds the data of its own by feedback of such data. It is thus rendered possible to prevent easy entry of the noise into the circuit.
FIG. 11 shows an example of an image forming apparatus capable of two-sided printing. An image forming apparatus 200, capable of two-sided printing by mounting a two-sided unit 168, is provided with a scanner unit 161, a photosensitive member 162 to be exposed to a laser unit emitted from the scanner unit 161, a developing unit 163 for developing a latent image formed by exposure on the photosensitive member 162, a transfer belt 165 for transferring an image onto a recording sheet 164, a fixing unit 166 for fixing the transferred image to the recording sheet 164, a cassette 167 holding a stack of the recording sheets 164, and an engine control unit 101 (to be explained later) for controlling these component units. Also the two-sided unit 168 for enabling two-sided printing on the recording sheet 164 is provided with a two-sided control unit 169 (to be explained later) for controlling such two-sided unit 168.
FIG. 12 is a block diagram showing connection of the engine control unit 101 and the two-sided unit control unit 169 by bidirectional clock-synchronized serial communication. Each of the engine control unit 101 and the two-sided unit control unit 169 emits and receives various unrepresented signals, and the engine control and the two-sided unit control are achieved by monitoring the states of these input signals. (In FIG. 12, there are only shown a communication synchronization clock CLK, transmission data T×D and reception data R×D.) In case a two-sided printing is instructed, the engine control unit 101 and the two-sided unit control unit 169 execute serial communication to share the controls whereby the two-sided unit control unit 169 controls the two-sided unit 168 while the engine control unit 101 controls other components. The engine control unit 101 is provided with a master CPU 102 including a serial communication unit 103 for executing clock synchronized communication with the two-sided unit control unit 169, while the two-sided unit control unit 169 is provided with a slave CPU 105 including a serial communication unit 106 for executing clock synchronized communication with the engine control unit 101.
In the following there will be explained, with reference to FIGS. 13A and 13B, a communication method of exchanging command and status by the serial communication unit 103 of the master CPU 102 and the serial communication unit 106 of the slave CPU 103.
FIG. 13A is a view showing the relationship between clock signal and data in the bidirectional clock synchronized communication. In synchronization with the downshift of a clock signal transmitted by the master CPU 102, the data transmitting side transmits data of 8 bits, from the LSB (least significant bit) to the MSB (most significant bit). On the other hand, in synchronization with the upshift of the clock signal transmitted by the master CPU 102, the receiving side receives the data of 8 bits from the LSB (least significant bit) to the MSB (most significant bit).
FIG. 13B is a timing chart showing the state of communication between the master CPU 102 and the slave CPU 105. In FIG. 13B, CLK, T×D and R×D are names from the side of the master CPU 102, and respectively indicate a communication synchronization clock, transmission data and reception data. At first, in synchronization with the downshift of the communication synchronization clock, the master CPU 102 transmits the transmission data T×D. Based on the SLK and T×D transmitted from the master CPU 102, the slave CPU 105 receives the reception data of 8 bits in synchronization with the upshift of the communication synchronization clock. Then the slave CPU 105 transmits the transmission data of 8 bits in synchronization with the downshift of the communication synchronization clock. The transmission data from the slave CPU 105 are the reception data R×D seen from the master CPU 102, which thus receives the reception data of 8 bits in synchronization with the upshift of the communication synchronization clock. In this manner the transmission data T×D and the reception data R×D are emitted and received in synchronization with the communication synchronization clock emitted by the master CPU 102. The engine control unit 101 and the two-sided unit control unit 169 are respectively provided with unrepresented input units for plural input signals, and an above-described noise elimination apparatus is required for such input unit in order not to capture erroneous digital data. Particularly in the bidirectional clock synchronized serial communication, if a noise is superposed on the communication synchronization clock signal, the slave side which receives the data in synchronization with the communication synchronization clock executes the data capture at a wrong timing, resulting in frequency reception errors. Also the reception data will involve a reception error if a noise is generated at the upshift of the communication synchronization clock. Similarly, also in the master side, there will result a reception error if a noise is generated at the upshift of the communication synchronization clock.
Thus, in the clock synchronized serial communication, a transmission error or a reception error will result in the data if a noise is superposed on the communication synchronization clock or on the transmission or reception data. Particularly if a noise is superposed on the communication synchronization clock signal, the slave side which receives the data in synchronization with the communication synchronization clock executes the data capture at a wrong timing, resulting in frequency reception errors. Also the reception data will involve a reception error if a noise is generated at the upshift of the communication synchronization clock.
In the clock synchronized serial communication, as the receiving side often recognizes erroneous data because of the noise, there has been adopted a measure as shown in FIG. 15. More specifically, in such measure, a low-pass filter consisting of a resistor 51 and a capacitor 52 is provided on each line connecting the engine control unit 101 and the two-sided unit control unit 169. Such low-pass filter has a cutoff frequency fc=1/(2πRC) and can attenuate and eliminate the noise of a frequency higher than such cutoff frequency.
In case the noise elimination apparatus of the variable digital noise filter type is provided on each of the input units for the plural digital inputs, as shown in FIG. 14, there will be required, corresponding to the number of the input units, the variable timing generation units for observing the state of the input signal plural times at a predetermined time interval and capturing the signal only if, after the change in the state of the signal, the changed state continues for a predetermined time, whereby the magnitude of circuitry becomes inevitably large.
Also in case data signal is transmitted from the slave side in synchronization with the clock signal outputted from the master side as in the bidirectional clock synchronized serial communication, the delay in time is accumulated, in the course of communication, by the forward signal (clock signal) and the returns signal (data signal), whereby the timing of receiving the data signal at the master side may be perturbed. Consequently, in case a delay in the communication is possible, the noise elimination apparatus of the variable digital noise filter type cannot be installed, so that the digital noise filter of high noise eliminating effect cannot be adopted. Stated differently, the conventional noise elimination apparatus of the variable digital noise filter type as shown in FIG. 10 has a wide application because the aforementioned predetermined time can be selected according to the desired level of noise elimination, but, because the aforementioned predetermined time can be arbitrarily selected, the reception timing for the data signal at the master side may be perturbed if such time is selected excessively large. For this reason, the variable digital noise filter has not been employed in practice, and the noise elimination has been achieved by the analog filter of a predetermined cutoff frequency fc as shown in FIG. 15. The cutoff frequency fc is determined as ½πRC by a preset resistor R and a preset capacitor C, whereby the noises of higher frequencies are eliminated by attenuation.